Multiprimary displays, consisting of at least four different colour sub-pixels, have been developed and produced in several different types, with several different performance aims.
Displays using more than the standard three (usually red, green and blue (RGB)) primaries for expansion of the range of displayable colours have been produced (Proceedings of the IDW'09, 2009, pp 1199-1202), as well as RGBW displays with an added white sub-pixel to improve display brightness and therefore efficiency (SID'08 Digest, pp 1112-1115). Multiprimary displays have also been produced with the aim of simultaneously increasing brightness, as well as increasing the ability to render fine image features on a sub-pixel level (IMID '05 Digest, pp 867-872). Displays with an additional yellow sub-pixel (RGBY) have also been developed possessing enhanced brightness, increased colour gamut, and increased sub-pixel rendering ability (SID'10 Digest, pp 281-282). As multiprimary displays have more than three types of colour sub-pixel, for many colour and luminance values, there may be multiple configurations of individual data values supplied to the colour sub-pixels which produce the target colour. The multiple configurations producing the same overall luminance and chromaticity are known as metamers. Selection of which of a set of possible metamers to use for a particular pixel in an image, based on sub-pixel rendering considerations, is described in US2010 0277498.
Several types of display which switchable between a public and private display mode, with varying degrees of additional cost over a standard display, ease of use and strength of privacy performance are also well known.
Devices incorporating such displays include mobile phones, Personal Digital Assistants (PDAs), laptop computers, desktop monitors, Automatic Teller Machines (ATMs) and Electronic Point of Sale (EPOS) equipment. Such devices can also be beneficial in situations where it is distracting and therefore unsafe for certain viewers (for example drivers or those operating heavy machinery) to be able to see certain images at certain times, for example an in car television screen while the car is in motion.
Several methods exist for adding a light controlling apparatus to a naturally wide-viewing range display, such as a microlouvre film (USRE27617 (F. O. Olsen; 3M 1973), U.S. Pat. No. 4,766,023 (S.-L. Lu, 3M 1988), and U.S. Pat. No. 4,764,410 (R. F. Grzywinski; 3M 1988)). However, this and other methods involving detachable optical arrangements are not conveniently switchable, requiring as they do manual placement and removal of the film or other apparatus to change the display from the public to the private mode.
Methods of providing an electronically switchable privacy function are disclosed in GB2413394 (Sharp), WO06132384A1 (Sharp, 2005) and GB2439961 (Sharp). In these inventions, a switchable privacy device is constructed by adding one or more extra liquid crystal layers and polarisers to a display panel. The intrinsic viewing angle dependence of these extra elements can be changed by switching the liquid crystal electrically in the well-known way. Devices utilising this technology include the Sharp Sh851i and Sh902i mobile phones. These methods share the disadvantages that the additional optical components add thickness and cost to the display.
Methods to control the viewing angle properties of an LCD by switching the single liquid crystal layer of the display between two different configurations, both of which are capable of displaying a high quality image to the on-axis viewer are described in US20070040780A1 and GB 0721255.8. These devices provide the switchable privacy function without the need for added display thickness, but require complex pixel electrode designs and other manufacturing modifications to a standard display.
One example of a display device with privacy mode capability with no added display hardware complexity is the Sharp Sh702iS mobile phone. This uses a manipulation of the image data displayed on the phone's LCD, in conjunction with the angular dataluminance properties inherent to the liquid crystal mode used in the display, to produce a private mode in which the displayed information is unintelligible to viewers observing the display from an off-centre position. A key advantage of this type of method is that in the public mode, the display consists of, and operates as, a standard display, with no image quality degradation causes by the private mode capability. However, when in the private mode, the quality of the image displayed to the legitimate, on-axis viewer is severely degraded.
Improved schemes which, when in the private mode, manipulate the image data in a manner dependent on a second, masking, image, and therefore causes that masking image to be perceived by the off-axis viewer when the modified image is displayed, are given in GB2428152A1, WO2009110128A1, WO201134209 and WO201134208. These methods provide an electronically switchable public/private display with no additional optical elements required, minimal additional cost, and satisfactory privacy performance. However, as the methods all utilise the limited resolution of the human visual system by representing the main and side images using the average luminance produced by groups of adjacent pixels. Therefore, a resolution loss in the image display to the On-axis viewer is incurred.
It is therefore desirable to provide a high quality LCD display which has public and private mode capability, in which no modification to the LC layer or pixel electrode geometry is required from a standard display, has a substantially unaltered display performance (brightness, contrast resolution etc) in the public mode, and in the private mode has a strong privacy effect with minimal degradation to the on-axis image quality, particularly with regard to resolution loss incurred in the private mode.